The present invention relates generally to an electron gun, and more in detail to an electron gun device suitable for a cathode ray tube such as a color picture tube and a color display tube. More specifically it relates to a color cathode ray tube using a single electron gun generating a plurality of beams.
Accompanied by enlargement of the screen and high definition of the image in a color picture tube device, a color cathode ray tube having a reduced aberration provided with a larger aperture electron lens is required. As a color cathode ray tube suitable for this requirement, a color cathode ray tube is known which as a single electron gun generating a plurality of beams. This electron gun has an electrode structure as indicated in FIG. 14 (disclosed e.g. in JP-B-Sho 49-5591). As indicated in the figure, for the cathodes K.sub.R, K.sub.G and K.sub.B arranged along the X-axis, corresponding to the different colors, red, green and blue, respectively, there are disposed in common a first electrode G.sub.1, a second electrode G.sub.2, a third electrode G.sub.3, a fourth electrode G.sub.4 and a fifth electrode G.sub.5. The cathodes, the first electrode G.sub.1 and the second electrode G.sub.2 constitute a triode part. The third electrode G.sub.3, the fourth electrode G.sub.4 and the fifth electrode G.sub.5, all of which are cylindrical, form a main electron lens of unipotential focusing type by applying a focusing voltage V.sub.G4 to the fourth electrode G.sub.4 and a same voltage V.sub.H to the third electrode G.sub.3 and the fifth electrode G.sub.5. The cathodes K.sub.R, K.sub.G and K.sub.B are so arranged that the electron beams therefrom intersect each other at a position, where the Fraunhofer condition (condition that the coma is zero) is satisfied approximately at the center of the main electron lens. Further the three electron beams B.sub.G, B.sub.R and B.sub.B are converged on the screen by electrostatic deflection plates A and B disposed in a stage succeeding the fifth electrode G.sub.5.
However, the uni-potential focusing lens has a drawback that if it is attempted to improve the aberration characteristics for the electron beam from the cathode K.sub.G, i.e. the center beam B.sub.G, the aberration characteristics are worsened for the electron beams from the cathodes K.sub.R and K.sub.B, i.e. the side beams B.sub.R and B.sub.B. This takes place for the reason described below. That is, in the uni-potential lens, since three electrodes are used as described above, it is possible to lengthen the acting region of the lens by increasing the length of the middle electrode. Therefore, it is possible to decrease the spherical aberration for the center beam B.sub.G by making the lens weaker by lengthening the acting region of the lens. However, since the side beams B.sub.R and B.sub.B enter the lens obliquely, if the acting region of the lens is long, a great astigmatism is produced, corresponding thereto. For this reason it is not possible to reduce the size of the spots for the side beams, even if the ratio of the electrode voltages is varied. That is, for a one-gun three-beam electron gun using the prior art uni-potential lens there was a limit of improving the aberration characteristics both for the center beam and for the side beams.
On the other hand, it is important also to decrease the depth of the color cathode ray tube.